The overall goal of this project is to understand and exploit the macroscopic effects of microscopic susceptibility variation in Nuclear Magnetic Resonance (NMR) imaging. Microscopic variations in local magnetic fields can produce large changes in NMR image contrast. This contrast can depend both on microscopic motions (proton diffusion) and macroscopic time course (passage of a bolus of contrast material, redistribution of iron oxide particles in the liver). the results of our preliminary studies suggest that the size and time scale of physiologically significant phenomena fall between the two previously studied limits, and thus present unique challenges for analysis and study. We seek to understand quantitatively a broad range of susceptibility phenomena: from the slowly varying equilibrium distribution of nano-sized superparamagnetic particles within the liver and spleen, to the intra-and extravascular effects of the rapid first-pass of susceptibility-inducing contrast agents through the cerebral and cardiac vasculature. We combine analytic modeling, Monte Carlo simulations, phantom and in vivo ultra-high speed imaging with novel pulse sequences to understand these effects, focusing in particular on clinical imaging field strengths. The results of this work should provide both an improved understanding of the use of susceptibility contrast in this new generation of functional NMR imaging techniques and a better understanding of the fundamental principles of susceptibility contrast.